Aqueous alkali metal halide brines are electrolyzed to yield chlorine and alkali metal hydroxide, e.g., caustic soda or caustic potash. One method of electrolysis producing an alkali metal hydroxide cell liquor is in an electrolytic cell having the anode separated from the cathode by a permionic membrane. Another method of electrolysis producing a cell liquor of alkali metal hydroxide and alkali metal chloride is in an electrolytic cell having a synthetic microporous diaphragm between the anode and the cathode.
In an electrolytic cell having the anolyte separated from the catholyte by a separator, alkali metal chloride brine is fed to the anolyte compartment and chlorine is evolved at the anodes. This gives rise to a froth of chlorine gas and depleted brine which is recovered from the cell, separated into gaseous chlorine and liquid brine fractions with the brine returned to the cell. Additionally, depleted brine may be recovered from the cell, resaturated, and returned to the cell. Alkali metal ion is transported through the synthetic separator to the catholyte compartment where hydrogen and alkali metal hydroxide are produced. Water may be added to the catholyte compartment to control the alkali metal ion content of the catholyte liquor, in this way controlling the efficiency of the cathode reaction.
The electrolytic cell may be in the form of one of a plurality of cells in a bipolar electrolyzer or the electrolytic cell may be monopolar cell. In a bipolar electrolyzer, a plurality of bipolar units are electrically and mechanically in series with the cathodes of one individual electrolytic cell and the anodes of the next adjacent electrolytic cell of the electrolyzer being mounted on a common structural unit, a bipolar unit. The bipolar unit includes a backplate having a catholyte-resistant member and an anolyte-resistant member.
The cathodic side of the bipolar unit contains a screen spaced from the steel backplate and defining a volume therebetween and hollow cathode fingers extending outwardly from the backplate. The volume within the hollow cathode fingers and the volume between the screen and the backplate define the catholyte volume.
The anodic side of the bipolar unit includes a valve metal backplate with coated valve metal fingers extending outwardly therefrom, substantially parallel to the cathode fingers. The adjacent bipolar units are assembled together to form an electrolytic cell with the anodes of one bipolar unit facing the cathodes of the next adjacent bipolar unit and substantially parallel thereto with a substantially uniform space, i.e., interelectrode gap, therebetween. Either a synthetic permionic membrane or a synthetic microporous diaphragm is positioned between the anode and cathode, dividing the cell into a catholyte compartment and an anolyte compartment.
A bipolar electrolyzer, as described hereinabove, may contain anywhere from two to a hundred or more individual electrolytic cells in the electrolyzer.
Alternatively, the electrolysis may be carried out in a monopolar cell. A monopolar cell has a cathodic half cell containing a screen spaced from an outside wall and defining a volume therebetween and hollow cathode fingers extending outwardly therefrom. The volume within the hollow cathode fingers and between the screen and backplate is the catholyte volume. The anodic element of the monopolar electrolyzer includes a valve metal coating or surface on an internal element of either a peripheral wall or the cell bottom and coated valve metal fingers extending outwardly therefrom. The anodic and cathodic half cells are assembled to form an electrolytic cell with the anodes facing the cathodes and substantially parallel thereto with a substantially uniform space, i.e., an interelectrode gap, therebetween. Additionally, a synthetic separator is positioned between the anode and cathode dividing the cell into a catholyte compartment and an anolyte compartment.
One problem encountered in electrolytic cells having synthetic separators is mounting the separator on an electrode. This becomes a critical problem when there are interleaved electrodes of complex shapes.
Synthetic separators, that is, synthetic halocarbon resins which may have acid groups thereon as exemplified by fluorocarbon resins with carboxylic acid groups, fluorocarbon resins with sulfonic acid groups, and fluorocarbon resins with various derivatives of the aforementioned groups as well as other groups, are difficult to join and require special assembly methods. These special assembly methods include chemical reactions at the laps and joints, heating, and compression.
According to the invention herein contemplated, the use of synthetic separators at electrolytically less active, complex shaped areas of the electrode are dispensed with thereby allowing the use of separators of simple shape. This is accomplished by providing electrolyte impermeable members at opposite ends of the electrode, to hold the permionic membrane in place. The electrolyte impermeable members may be the cell top and cell bottom or they may be flanges or the like held in compression at opposite ends of the electrode.